1. Field of the Invention
The present invention relates to an image forming apparatus. Particularly, the present invention relates to an image forming apparatus having a transfer belt tightly stretched by a driving roller and a tension roller.
2. Description of the Related Art
Electrophotographic image forming apparatuses are broadly classified into direct-transfer type image forming apparatuses in which a toner image formed on a surface of an image bearing member such as photoreceptors is directly transferred onto a receiving material such as paper sheets, and indirect-transfer type image forming apparatuses in which one or more toner images formed on surfaces of multiple image bearing members are primary transferred onto an intermediate transfer belt, and the toner images are then transferred onto a receiving material. The indirect-transfer type image forming apparatuses are typically used as multi-color image forming apparatuses (such as full color image forming apparatuses).
Multi-color image forming apparatuses using an intermediate transfer belt typically include the following image forming processes:    (1) a toner image forming process in which different color toner images are formed on plural image bearing members such as photoreceptors;    (2) a primary transfer process in which the color toner images are sequentially transferred onto a surface of an intermediate transfer belt to form a combined multi-color toner image on the intermediate transfer belt;    (3) a secondary transfer process in which the combined multi-color toner image is transferred onto a receiving material; and    (4) a fixing process in which the combined multi-color toner image is fixed to the receiving material, resulting in formation of a multi-color image.
Direct-transfer type multi-color image forming apparatuses typically include the following image forming processes:    (1) a toner image forming process in which different color toner images are formed on plural image bearing members such as photoreceptors;    (2) a transfer process in which the color toner images are sequentially transferred onto a receiving material fed by a direct-transfer belt similar to such an intermediate transfer belt as mentioned above while contacted therewith to form a combined multi-color toner image on the receiving material; and    (4) a fixing process in which the combined multi-color toner image is fixed to the receiving material, resulting in formation of a multi-color image.
Hereinafter, an intermediate transfer belt and a direct-transfer belt are referred to as a transfer belt in this application.
Such a transfer belt is typically tightly stretched by plural rollers (such as a combination of a driving roller and a tension roller) while a tension is applied to the transfer belt using a member such as a tension roller, so that each of the plural rollers does not cause slipping and an image misalignment problem, which is caused by the slipping and in which different color toner images on image bearing members are not transferred to the desired positions of the transfer belt or a receiving material, resulting in formation of a misaligned color image.
In addition, there are image forming apparatuses having a transfer belt stretched by a combination of a driving roller and a movable roller, which serves as a tension roller and is movable in such a direction that the interval between the driving roller and the movable roller is changed, to miniaturize the image forming apparatuses.
Some of such image forming apparatuses include a detector, which is provided so as to be opposed to a tension roller to optically detect a toner image formed on a transfer belt to adjust transfer positions of the following toner images and to prevent occurrence of the image misalignment problem, or to optically detect marks formed on a transfer belt at regular intervals to control the moving speed of the transfer belt. For example, there is an image forming apparatus in which an image reading sensor serving as a detector is provided on a holding member holding a tension roller supporting a transfer belt so that the sensor is opposed to the tension roller with the transfer belt therebetween.
In this image forming apparatus, the shaft of the tension roller and the image reading sensor are held by the holding member. Therefore, if the holding member is damaged (for example, there is a case in which when the unit is detached from the image forming apparatus to replace the transfer belt, the holding member mistakenly strikes against the main body of the image forming apparatus), the detection accuracy of the sensor deteriorates due to change of the position of the light spot formed by the sensor and the distance between the sensor and the transfer belt. In addition, it is necessary for the holding member to have good dimensional accuracy so that the accuracy of the sensor can be satisfactorily shown.
For these reasons, the present inventors recognized that there is a need for an image forming apparatus which includes a transfer belt tightly stretched by a driving roller and a tension roller, and a detector optically detecting a mark (such as a toner image) on a surface of the transfer belt, wherein the detector is accurately positioned relative to the tension roller so that the position of a light spot formed by the detector is hardly changed and thereby the mark on the transfer belt can be accurately detected by the detector.